1. Field of the Invention
The invention relates to a semiconductor structure, and more particularly to a composite polycrystal substrate for growth of a single crystal layer.
2. Description of the Related Art
Recently, there has been enormous interest in the growth of Group III nitrides, and particularly gallium nitride (GaN) thin films, Jpn. J. Appl. Phys. Vol. 34 (1995) pp. L 797-L 799. GaN, and related (Aluminum, Indium) N alloys are being utilized for the production of efficient optoelectronic devices, such as, light emitters and detectors spanning the spectral range of visible to deep ultra-violet (UV) light. The direct, wide bandgap and the chemical stability of Group III nitrides are very beneficial for high-temperature and high-power electronic devices, such as, hetero-junction bipolar and field effect transistors.
When GaN is directly grown on a sapphire substrate, the growth mode is three-dimensional due to the large lattice mismatch, the chemical dissimilarity, and the thermal expansion difference. The layer contains structural defects such as point defects, misfit dislocations, and stacking faults. These defects degrade the structural, morphological, and electronic properties of the film. In order to achieve high quality epitaxial growth, researchers have introduced a thin low-temperature grown AlN or GaN layer serving as a buffer layer. This layer provides nucleation sites for subsequent two-dimensional GaN growth at higher temperatures, see H. Amano, M. Kito, K. Hiramatsu, and I. Akasaki, Jpn. J. Appl. Phys. 28, L2112 (1989) and S. Nakamura, T. Mukai, M. Senoh, and N. Isawa, Jpn. J. Appl. Phys. 31, L139 (1992). Thus, control of buffer layer growth is the most important step in improving GaN layer properties. The effect of buffer layer thickness and growth temperature on GaN layer properties has been researched by G. S. Sudhir, Y. Peyrot, J. Kruger, Y. Kim, R. Klockenbrink, C. Kisielowski, M. D. Rubin and E. R. Weber, Mat. Res. Symp. Proc. 482, pp. 525-530 (1998); Y. Kim, R. Klockenbrink, C. Kisielowski, J. Kruger, D. Corlatan, Sudhir G. S., Y. Peyrot, Y. Cho, M. Rubin, and E. R. Weber, Mat. Res. Symp. Proc. 482, pp. 217-222 (1998); J. Kruger, Sudhir G. S., D. Corlatan, Y. Cho, Y. Kim, R. Klockenbrink, S. Rouvimov, Z. Liliental-Weber, C. Kisielowski, M. Rubin and E. R. Weber, and results published in Mat. Res. Symp. Proc. 482 pp. 447-452 (1998). Buffer layers for Group-III nitride growth has been discussed in Mohammad et al., “Progress and Prospects of Group-III Nitride Semiconductors”, Prog. Quant. Electr. 1996, Vol. 20, No. 5/6 pp. 418-419, hereby incorporated by reference in its entirety. Various buffer materials are disclosed.
GaN and related alloys are of particular interest for light emitting thin films because of their ability to cover a wide spectral range. Because a crystalline substrate with a lattice parameter near that of GaN is not yet available, different growth techniques are desirable in order to limit defect density.